In electron tubes, such as traveling-wave tubes (TWTs), it is necessary to provide a focusing field for the electron stream as it travels along the tube, from the cathode to the collector. The focusing field, be it magnetic or electrostatic, must be of a strength appropriate to overcome the space-charge forces within the electron tube that would otherwise cause the electron beam to spread. In the past a longitudinal magnetic field was supplied along the length of the electron tube utilizing a electromagnetic solenoid. However, the continuing demands for improved efficiency and reliability, and for weight and size reduction, have resulted in the development of periodic permanent magnet (PPM) structures. As will be recognized by a person skilled in the art, PPM structures focus the electron beam by periodically positioning magnets of opposite polarity along the length of the electron tube, thereby creating a periodically reversing magnetic field which acts to confine the passing electron beam.
Typically, in prior art PPM assemblies, a series of angularly formed pole pieces, non-magnetic spacers and individual ring magnets are stacked on top of one another to form an elongated cylinder in which a linear or semi-linear electron beam device can be placed. In such prior art PPM assemblies, the pole pieces and non-magnetic spacers are fabricated as cylindrical sections, which are joined to create the overall cylindrical shape of the PPM assembly. Typically, the ring magnets are formed as semi-circles and are affixed to either side of the various pole pieces by being either clamped, taped or glued into place. The process joining the pole pieces to the non-magnetic spacers and affixing the ring magnets to the pole pieces, results in an assembly procedure that is inefficient, requiring excessive handling of the PPM assembly and long assembly time.
It is therefore a primary objective of the present invention to set forth a PPM assembly and corresponding method that is both less expensive and less labor intensive to assemble, thereby reducing the cost of manufacturing the PPM assembly and reducing damage to the PPM assemblies caused by excessive handling.